From the earliest times, Crests have occasionally been identical with the principal charge in the Shield of Arms, or they have repeated the principal charge with some slight modification of attitude or accessory: but, more generally, Crests have been altogether distinct.

From The Handbook to English Heraldry by Charles Boutell, p246

Accordingly, about the time that Coat-Armour became hereditary, having been reduced to a system and accepted as an independent science, heraldic Crests began to be worn as honourable distinctions of the most exalted dignity by the mediaeval chivalry.

From The Handbook to English Heraldry by Charles Boutell, p243